1. Field of the Invention
The present invention relates to a piezoelectric device having a piezoelectric body and a characteristic crystalline structure, and a process for driving the piezoelectric device under an appropriate condition. The present invention also relates to a piezoelectric actuator having the above piezoelectric device and a means for controlling the piezoelectric device, and a liquid discharge device using the piezoelectric device.
2. Description of the Related Art
Currently, piezoelectric devices constituted by a piezoelectric body and electrodes are used as, for example, actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric body expands and contracts according to increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body along a predetermined direction. For example, complex compounds having a perovskite structure such as PZT (lead titanate zirconate) are known as materials suitable for the piezoelectric body. Such materials are ferroelectric materials which exhibit spontaneous polarization even when no electric field is applied to the materials.
The conventional piezoelectric devices commonly utilize only the so-called piezoelectric effect. The piezoelectric effect is a phenomenon in which a piezoelectric body expands along a polarization axis when an electric field is applied to the piezoelectric body along the polarization axis. (Hereinafter, the conventional piezoelectric devices utilizing only the above piezoelectric effect are referred to as the first type of conventional piezoelectric devices.) Therefore, it has been conventionally considered important that the material of the piezoelectric body be designed so that the orientation of the polarization axis coincides with the direction of the applied electric field.
However, the magnitude of strain which can be achieved by use of only the conventional piezoelectric effect as in the first type of conventional piezoelectric devices is limited, so that demands for greater strain are increasing.
On the other hand, since the size and weight of the electronic devices are decreasing and the functions of the electronic devices are being sophisticated, development of the piezoelectric devices for reducing the size and weight of the piezoelectric devices and improving the functions of the piezoelectric devices are proceeding. For example, in the field of the inkjet recording heads, techniques for increasing the density of arrangement of piezoelectric devices are currently being studied in order to improve image quality. For this purpose, techniques for reducing the thicknesses of the piezoelectric bodies in the producing piezoelectric devices are also being studied. Even when the magnitude of the voltage applied to a piezoelectric body is unchanged, the strength of the electric field applied to the piezoelectric body is increased with decrease in the thickness of the piezoelectric body. Therefore, sufficient piezoelectric performance cannot be achieved by the conventional material design.
As conventionally known, in the case where only the conventional piezoelectric effect contributes to the strain in a ferroelectric substance (as in the first type of conventional piezoelectric devices), the piezoelectric characteristic of the ferroelectric substance (i.e., the relationship between the strength of the applied electric field and the magnitude of strain in the ferroelectric substance) is as indicated by the thick solid curve Q in FIG. 6. The curve Q indicates that the magnitude of strain in the ferroelectric substance linearly increases with increase in the strength of the electric field (i.e., the electric field strength) until the electric field strength reaches a certain level Ex. However, when the electric field strength exceeds the certain level Ex, the increase in the magnitude of strain is greatly reduced and the magnitude of strain is almost saturated.
Therefore, conventionally, the first type of conventional piezoelectric devices have been used within the range of the electric field strength from 0 to Ex (in which the magnitude of strain in the ferroelectric substance linearly increases with increase in the electric field strength). Although the level Ex of the electric field strength and the maximum electric field strength depend on the material (composition) of the ferroelectric substance, for example, the level Ex of the electric field strength is approximately 5 to 100 kV/cm, and the maximum electric field strength is conventionally approximately 0.1 to 10 kV/cm. However, in the case where the thickness of the piezoelectric device is reduced, the electric field strength applied to the piezoelectric body increases even when the magnitude of the voltage applied to the piezoelectric device is unchanged. Therefore, the piezoelectric devices are obliged to be used, for example in the range of the electric field strength from 0 to Ey, where Ey>Ex as indicated in FIG. 6. In this case, the effective piezoelectric constant in the range of the electric field strength from 0 to Ey (which is indicated by the gradient Q′ in FIG. 6) is smaller than the piezoelectric constant in the range of the electric field strength from 0 to Ex, so that the piezoelectric performance which the piezoelectric devices intrinsically have is not sufficiently delivered.
In particular, in the case where a piezoelectric device the thickness of which is reduced is used in such a manner that the difference between the minimum electric field strength and the maximum electric field strength is unchanged from the conventional piezoelectric devices the thickness of which is not reduced, for example, in the case where the piezoelectric device with the reduced thickness is used in the range from Ex to Ey, the magnitude of strain can vary by only a very small amount, so that the piezoelectric device cannot sufficiently deliver the function which the piezoelectric device is required to have.
In the above circumstances, the Japanese Patent No. 3568107 (hereinafter referred to as JP3568107) proposes a second type of conventional piezoelectric device, in which application of an electric field causes phase transition in a piezoelectric body. JP3568107 discloses a piezoelectric device constituted by a phase-transition film, electrodes, and a heating body, where the heating body adjusts the temperature of the phase-transition film to a level near to the Curie point Tc. (See “claim 1” in JP3568107.) JP3568107 discloses as the phase-transition film a film in which transition occurs between a tetragonal phase and a rhombohedral phase or between a cubic phase and a tetragonal or rhombohedral phase. (See “claim 2” in JP3568107.) Further, JP3568107 reports that the second type of conventional piezoelectric device disclosed in JP3566107 can achieve greater strain than the first type of conventional piezoelectric devices because both of the piezoelectric effect of the ferroelectric material and the change in the crystal structure associated with the phase transition contribute to the magnitude of strain in the second type of conventional piezoelectric device.
As explained above, JP3568107 refers to the use, as the phase-transition film, of a film in which phase transition occurs between a tetragonal system and a rhombohedral system, and a film in which phase transition occurs between a cubic system and a tetragonal or rhombohedral system, where both of the tetragonal and rhombohedral phases exhibit ferroelectric properties, and the cubic phase exhibit paraelectric properties. However, the piezoelectric device disclosed in JP3568107 is supposed to be used in the vicinity of the Curie point Tc. Since the Curie point Tc corresponds to the phase-transition temperature between the ferroelectric phase and the paraelectric phase, the phase transition between the tetragonal phase and the rhombohedral phase occurs in no film used in the vicinity of the Curie point Tc. That is, the piezoelectric device disclosed in JP3568107 cannot take advantage of the phase transition other than the phase transition between the ferroelectric phase and the paraelectric phase. In addition, since spontaneous polarization does not occur in the paraelectric material, the piezoelectric device disclosed in JP3568107 does not exhibit the piezoelectric effect of expansion in the direction of the polarization in response to application of an electric field after the phase transition.
Further, FIG. 6 also schematically indicates the piezoelectric characteristic of the second type of conventional piezoelectric device by the curve R. In order to clarify the difference of the piezoelectric device according to the second type of conventional technique from the first type of conventional piezoelectric device, the essentially identical portions of the piezoelectric characteristics of the first and second types of conventional piezoelectric devices in the range of the electric field strength from 0 to E1 are indicated by the common solid line, which is a portion of the curve Q, where the phase transition begins at the electric field strength E1. That is, the conventional piezoelectric effect makes the magnitude of strain in the piezoelectric device linearly increase with increase in the electric field strength until the phase transition begins at the electric field strength E1. In the range of the electric field strength from E1 to E2, the change in the crystal structure makes the magnitude of strain increase with increase in the electric field strength, where the phase transition to the paraelectric phase is substantially completed at the electric field strength E2. In the range of the electric field strength above E2, the magnitude of strain does not increase with increase in the electric field strength since the piezoelectric effect of the ferroelectric substance does not work in the paraelectric phase.
In the case where the thickness of the second type of conventional piezoelectric device is reduced, the second type of conventional piezoelectric device is also obliged to operate in the range of the electric field strength which includes the range in which the electric field strength is high and the magnitude of strain does not increase with increase in the electric field strength. Therefore, the second type of conventional piezoelectric device cannot effectively deliver the function which the piezoelectric device is required to have, as the first type of conventional piezoelectric devices.